1. (regexp, RE) One of the wild card patterns used by Perl and other languages, following
Unix utilities such as grep, sed, and awk and editors
such as vi and Emacs. Regular expressions use conventions
similar to but more elaborate than those described under
glob. A regular expression is a sequence of characters with
the following meanings:

An ordinary character (not one of the special characters
discussed below) matches that character.

A backslash (\) followed by any special character matches the
special character itself. The special characters are:

"." matches any character except NEWLINE; "RE*" (where
the "*" is called the "Kleene star") matches zero
or more occurrences of RE. If there is any choice, the
longest leftmost matching string is chosen, in most
regexp flavours.

"
" at the beginning of an RE matches the start of a line and
"$" at the end of an RE matches the end of a line.

[string] matches any one character in that string. If the
first character of the string is a "
" it matches any
character except the remaining characters in the string (and
also usually excluding NEWLINE). "-" may be used to indicate
a range of consecutive ASCII characters.

\( RE \) matches whatever RE matches and \n, where n is a
digit, matches whatever was matched by the RE between the nth
\( and its corresponding \) earlier in the same RE. Many
flavours use ( RE ) used instead of \( RE \).

The concatenation of REs is a RE that matches the
concatenation of the strings matched by each RE. RE1 | RE2
matches whatever RE1 or RE2 matches.

\< matches the beginning of a word and \> matches the end of a
word. In many flavours of regexp, \> and \< are replaced by
"\b", the special character for "word boundary".

2. Any description of a pattern composed from combinations
of symbols and the three operators:

Concatenation - pattern A concatenated with B matches a match
for A followed by a match for B.

Or - pattern A-or-B matches either a match for A or a match
for B.

Closure - zero or more matches for a pattern.

The earliest form of regular expressions (and the term itself)
were invented by mathematician Stephen Cole Kleene in the
mid-1950s, as a notation to easily manipulate "regular sets",
formal descriptions of the behaviour of finite state machines, in regular algebra.